Most prior art RF photonic links comprise a single laser, a single modulator and a single photodetector. In such prior art RF photonic links, a way to increase the signal-to-noise ratio (SNR) is to increase the laser power that is delivered to the modulator. The increase in SNR is proportional to the increase in laser power so long as the noise of the link is dominated by the shot noise processes in the photodetector. However, when the laser power becomes sufficiently high, the noise becomes dominated by the intensity noise of the laser. In that case, additional increases in the laser power do not result in further improvement in the SNR. There is a need to further increase the SNR. The present invention accomplishes this by using multiple modulators and combining their modulated outputs. The use of RF photonic links that comprise multiple sets of laser, modulator and photodetector was not as beneficial in the past because the input RF signal would need to be divided among those multiple modulators. Thus, the depth-of-modulation achieved in each of those sets would be reduced accordingly, and the photodetector output signal of each set also would be reduced. In contrast, the present invention applies the same RF input to the multiple modulators, without needing to divide that RF power. Since the depth-of-modulation for each set is not compromised, a combination of the modulated light from those multiple sets can yield higher photodetected output signal.
The circuits of this disclosure achieve greater signal-to-noise ratio (SNR) compared to an RF photonic link that contains only a single modulator. For an RF photonic link according to this disclosure that has one photodetector and multiple lasers of differing wavelengths, the improvement in the photodetected signal power is proportional to N, the number of modulators (and the number of lasers), assuming each laser emits light of the same power. The DC component of the light delivered to the photodetector also is proportional to N. In that case, the link preferably is operated at a laser power wherein the laser intensity noise dominates the photodetector shot noise. The intensity noise contributions from different lasers are uncorrelated. The noise power increases as the square root of N. Thus the improvement in SNR, compared to the SNR of an RF photonic link having a single modulator is proportional to the square root of the number of modulators in the multiple-modulator RF photonic link.
For a link with multiple photodetectors whose RF outputs are coupled together using phase-sensitive couplers or at a current-summing node, again the improvement in the photodetected signal power is proportional to N, the number of modulators (and the number of lasers), assuming each laser emits light of the same power. For such an RF photonic link, the shot noise contributions from the multiple photodetectors are uncorrelated. Thus, this RF photonic link can be operated at a laser power wherein the photodetector shot noise dominates the laser intensity noise. In that case, the link can have a single laser supply the light to the multiple modulators.
However, if the link is to be operated at a laser power wherein the laser intensity noise is dominant, that link should preferably have multiple lasers as well as multiple photodetectors, so that the dominant noise contributions will be uncorrelated. The improvement in SNR again can be a great as the square root of the number of modulators.
There are a number of military and commercial applications of RF photonic links. These applications include fiber radio in which signals for wireless RF networks (such as cell phone networks) are transported to/from the base stations through optical fiber. These signals have limited bandwidth but are at high carrier frequency, with the carrier frequency ranging from 1 to 60 GHz. The bandwidth of these signals is generally less than 5-10% of the carrier frequency. The signal bandwidth for defense applications can be even larger. The enhanced modulation depth of this invention is well suited both to these commercial applications and to defense applications. The integrated RF waveguide and modulators of this invention can be coupled directly to an RF antenna or some other EM field concentrating structure.
An exemplary prior art modulating device having two parallel modulators is described in an article by Bridges and Schaffner (IEEE Transactions on Microwave Theory and Techniques, vol. 43, no. 9, September 1995, pp. 2184-2197) and shown in FIG. 1 herein. Two modulators are optically arranged in parallel, with the light supplied to them split by means of an optical directional coupler into two paths. One of those paths goes to the first modulator and the other of those paths goes into the second modulator. The RF electric fields for modulating the refractive index in these two modulators are provided by two different coplanar-waveguide RF transmission lines, which share only a common ground electrode. These two transmission lines have separate signal electrodes, with one signal electrode being associated with the first modulator and a second signal electrode being associated with the second modulator. With this prior art device, an input RF signal must be divided or split into two paths with one path directed to the first signal electrode and the other path directed to the second signal electrode. Thus, the input RF power is divided among these two paths so the power driving each of the two modulators is reduced. In contrast to this prior art, the present invention does not divide the input RF power but rather supplies the entire input RF power to each of its multiple modulators.
A prior art RF photonic link that comprises a parallel connection of multiple optical modulators is described in U.S. Pat. No. 6,724,523, whose inventor, D. Yap, is the inventor of the present application. U.S. Pat. No. 6,724,523 is hereby incorporated herein by reference. This prior art RF photonic link is illustrated in FIG. 2 herein. Light from one laser 102 is divided into multiple optical paths and thereby supplied to multiple optical modulators 106. Also, the RF drive signal is divided into multiple electrical paths and thereby supplied to those multiple optical modulators. The modulated light is supplied to multiple photodetectors 302 whose electrical output currents are combined together into a common load-impedance RLOAD. Some embodiments of the present invention use the same approach as used in this prior art photonic link for combining the electrical outputs of multiple photodetectors. However, unlike this prior art, the present invention does not divide the input RF power but rather supplies the entire input RF power to each of the multiple modulators.
A prior art optical modulator that contains an RF electrode structure wherein the RF field feeds multiple modulating sections in a series or cascaded manner is described in U.S. Pat. No. 7,260,280. In this device (illustrated in FIG. 3) the light to be modulated propagates in the same direction as does the RF field propagating in the electrode structure. Also, the multiple portions of optical modulator that are driven by the series connection of electrodes are themselves optically arranged in series. In contrast, the present invention connects the multiple distinct optical modulators in parallel.
In these prior art photonic links, the RF field propagating in the electrodes of the modulators travels in the same direction as the optical field propagating through those modulators (with that optical field being modulated because of the presence of the RF field). A novel feature of the present invention is that the RF field propagating in its RF waveguide travels in a direction that is perpendicular to the direction in which the optical field propagates through the modulators. This perpendicular relationship allows the propagating RF field to traverse multiple optoelectronic modulators sequentially. Although the RF waveguide provides a “traveling-wave” electrode, the drive electrode for each optoelectronic modulator is in essence a “bulk” electrode (i.e., the RF drive signal for the entire length of that optoelectronic modulator arrives at the same instant in time). This dual use of an RF waveguide as both a traveling-wave structure for the RF field and as a bulk electrode for the modulator, with the bulk electrode not being a termination of the traveling-wave structure, also is different from prior art, which typically places the bulk electrode at the termination of an RF cable.